In 1833, opened an office on Liberty Avenue the same year. He was also the owner of an apothecary on Wood Street. Kerr worked as a physician until he was elected.[1][2]
Kerr was a Democrat. He was rejected for the Democratic nomination for Mayor of Pittsburgh in 1844. He defeated incumbent mayor William J. Howard in a close election in 1845. He served as mayor from January 13, 1846, to January 2, 1847. During his administration, Pittsburgh was being rebuilt with brick and stone following the Great Fire of Pittsburgh in 1845. During his time in office, he advocated for the merger of Allegheny and Pittsburgh. He did not seek re-election in 1847, but ran again in 1848 and lost by 96 votes.[1][2]
Kerr married Mary Warden on March 26, 1840. They had four children.[1][2] A son, also named William, was a co-owner of the Pittsburgh Pirates during the 1890s.[3] His sister-in-law was the daughter of Pittsburgh mayor John M. Snowden.[2]
In 1848, Kerr moved to Chartiers Township, which is now the West End of Pittsburgh.[2]
Kerr died on August 11, 1853, at the age of 43. He was buried at Allegheny Cemetery.[2]